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iigs-game-engine
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Optimize odd-aligned case in the blitter 

The SEP/REP pairs that are used to move in and out
of 8-bit mode to do the single-byte pushed on the left
and right edges of the screen can also be used to clear
the necessary carry and overflow flags.
This commit is contained in:
Lucas Scharenbroich 2021-11-22 16:13:29 -06:00
parent 4075ed911c
commit 140126afd8
1 changed files with 15 additions and 17 deletions
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32
src/blitter/Template.s
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@ -38,7 +38,7 @@ PagePatches da {long_0-base+2}
da {loop_exit_3-base+2}
da {even_exit-base+2}
da {jmp_rtn_1-base+2}
da {jmp_rtn_2-base+2}
; da {jmp_rtn_2-base+2}
]index equ 0
lup 82 ; All the snippet addresses. The two JMP
@ -526,7 +526,7 @@ long_1 stal *+4-base ; Everything else is a two-b
r_is_pea xba ; fast code for PEA
r_jmp_rtn sep #$20 ; shared return code path by all methods
pha
rep #$20
rep #$61 ; Clear Carry, Overflow and M bits #$20
odd_entry jmp $0100 ; unconditionally jump into the "next" instruction in the
; code field. This is OK, even if the entry point was the
; last instruction, because there is a JMP at the end of
@ -549,6 +549,18 @@ long_3 stal *+5-base
; Return to caller -- the even_exit JMP from the previous line will jump here when a render is complete
full_return jml blt_return ; Full exit
; The even/odd branch of this line's exception handler will return here. This is mostly
; a space-saving measure to allow for more code in the exeption handers themselved, but
; also simplified the relocation process since we only have to update a single address
; in each exception handler, rather than two.
;
; Oce working, this code should be able to be interleaved with the r_jmp_rtn code
; above to eliminate a couple of branches
jmp_rtn
bvs r_jmp_rtn
jmp_rtn_1 jmp l_jmp_rtn-base ; Could inline the code and save 3 cycles / line
; Re-enable interrupts and continue -- the even_exit JMP from the previous line will jump here every
; 8 or 16 lines in order to give the system time to handle interrupts.
enable_int ldal stk_save+1 ; restore the stack
@ -566,20 +578,6 @@ enable_int ldal stk_save+1 ; restore the stack
rep #$20
bra entry_1
; The even/odd branch of this line's exception handler will return here. This is mostly
; a space-saving measure to allow for more code in the exeption handers themselved, but
; also simplified the relocation process since we only have to update a single address
; in each exception handler, rather than two.
;
; Oce working, this code should be able to be interleaved with the r_jmp_rtn code
; above to eliminate a couple of branches
jmp_rtn
bvs jmp_rtn_v ; overflow set means this is the right edge (entry)
clc ; carry is set only for edge operations; force clear
jmp_rtn_1 jmp l_jmp_rtn-base
jmp_rtn_v rep #$41 ; clear V and C
jmp_rtn_2 jmp r_jmp_rtn-base
; This is the spot that needs to be page-aligned. In addition to simplifying the entry address
; and only needing to update a byte instad of a word, because the code breaks out of the
; code field with a BRA instruction, we keep everything within a page to avoid the 1-cycle
@ -622,7 +620,7 @@ long_4 stal *+4-base
l_jmp_rtn xba
sep #$20
pha
rep #$20
rep #$61 ; Clear everything C, V and M
bra even_exit
l_is_jmp sec ; Set the C flag (V is always cleared at this point) which tells a snippet to push only the high byte